1. Field of the Invention
The present invention relates to a disc playback apparatus and method for reproducing information recorded on a disc-shaped recording medium, and, particularly, to a disc playback apparatus, e.g., a compact disc (CD) drive, and method for reproducing information based on a radio-frequency (RF) signal obtained by receiving reflected light of a light beam applied to a recording surface of a recording medium.
More specifically, the present invention relates to a disc playback apparatus and method for reproducing information from a recording surface of removable discs of different types that are removably loaded in the disc playback apparatus, such as a CD-recordable (CD-R) disc and a CD-rewritable (CD-RW) disc. In particular, the present invention relates to a disc playback apparatus and method for reproducing information without errors even if the type of disc loaded in the disc playback apparatus changes.
2. Description of the Related Art
With the recent technological innovations, various types of calculation systems have been developed and become commercially available. In general, calculation systems include an external storage system for storing necessary information or installing an operating program.
One typical storage device is a magnetic recording hard disk drive. Several magnetic media are accommodated as recording media in the drive unit, and each medium is rotated at a high rate by a spindle motor. Each medium is coated with a magnetic material that is plated with nickel phosphorous or the like. Scanning of a magnetic head on a surface of a rotating medium along the radius of the rotating medium causes magnetization on the medium to write or read data.
The use of hard disks has already become widespread. For example, hard disks are used as standard external storage devices for personal computers, and are used for installing various software programs, such as operating system (OS) programs necessary for booting the computers and utility application programs, or storing created or edited files.
Another type of external storage device is configured such that a removable recording disc for transferring an operation program to be installed on a hard disk or data content among a plurality of systems is removably loaded into the device to reproduce information. In the past, portable media, such as flexible discs, were the mainstream. Recently, with the increasing size of programs and data content, disc-shaped recording media with a relatively large capacity, such as CDs, and apparatuses for playing back such discs have increasingly become popular.
A CD is a recording medium from which information can be reproduced based on an RF signal obtained by receiving reflected light of a light beam applied to a recording surface of the disc. Recently, not only read-only CDs (“press CDs”) in which information is recorded as pits by manufacturers, such as compact disc read-only memory (CD-ROM) discs, but also discs on which information is recordable by end users, such as CD-R and CD-RW discs, have become available, and drives compatible with such user-recordable discs have increasingly become popular.
There are a variety of CDs depending on the combination of the reflective layer and the dye of the recording layer (see, for example, Yasuhiro MORI, “CD-R/DVD-R Masuta (CD-R/DVD-R Master)”, Sansai Mook, pages 90 to 92, Jun. 1, 2002). For example, a CD-R disc is composed of a substrate, an organic dye layer, a recording layer, a reflective layer, an overcoat layer, and a protective layer. The recording layer is formed with dyes, e.g., cyanine blue dye, phthalocyanine dye, or azo dye, depending on the disc manufacturer. The reflective layer is one of two colors, either gold or silver. Data is written on a CD-R disc by creating pits with a semiconductor laser beam with a wavelength of 780 nanometers. Specifically, the dye decomposes with heat of the laser beam, and the substrate of polycarbonate is exposed to heat and is grooved to produce pits. Once data has been recorded, the recorded data is not erasable. A press CD has a similar structure to a CD-R disc, but has no dye layer because the reflective layer is directly deposited by aluminum evaporation on a polycarbonate substrate that is preformed with lands and grooves.
A CD-RW disc is composed of a substrate, a reflective layer, a lower protective layer, a phase-change recording layer, a reflective layer, an upper protective layer, an ultraviolet (UV) coating layer, and a printing surface. The phase-change recording layer is rapidly heated with the energy delivered by a laser beam, and is then rapidly cooled to produce amorphous areas, thereby forming pits. These pits reflect less light than the remaining crystalline areas. Erasing is performed by rapidly heating the recording layer with the energy delivered by the laser beam, which is of a temperature between those for recording and playback, and, then, slowly cooling it to return the amorphous state back to the crystalline state.
As discussed previously, a recording surface of such CDs is irradiated with a laser beam, and the light reflected from the recording surface is received to produce an RF signal, based on which information is reproduced. When the RF signal is not reproduced under good conditions, it is difficult to perform high-quality digital signal processing.
A CD player designed for music reproduction or a CD-ROM player designed for computer-data reproduction is not acceptable as a system when data errors occur in excess of a predetermined amount. Data errors are correctable by a signal processing circuit or the like. However, if the RF signal reproduced from a disc does not have sufficiently high quality, errors are not fully corrected for by the correction ability of the signal processing circuit, and data errors occur.
When a disc of the CD or CD-ROM format is played back, errors may not be fully corrected for due to low quality of the RF reproduction signal waveform (the eye pattern for 3T to 11T), and the reproduced sound may be contaminated with noise or data errors may occur. It is therefore necessary to perform frequency correction depending on the reproduced RF signal to facilitate digital signal processing.
In order to improve the quality of an RF reproduction signal of a disc of the CD or CD-ROM format, high-frequency compensation is generally performed on the RF reproduction signal. An RF high-frequency-compensation equalizer circuit or the like is typically used as a compensator at the stage before performing signal processing on a low-quality RF reproduction signal that is produced by a combination of the disc and an optical pickup, etc.
There has been proposed, for example, an optical disc device including an equalizer for changing a gain-frequency characteristic of an RF signal depending on the received light level of the light reflected from a disc that is irradiated with a laser beam, in which the characteristic of the equalizer is changed depending on the received light level, thereby reproducing information without errors irrespective of the disc type (see, for example, Japanese Unexamined Patent Application Publication No. 2001-14680, FIG. 2). This optical disc device controls the rise characteristic of the RF signal so as to be constant by increasing the gain in the high-frequency band when the received light level is low and by decreasing the high-frequency gain when the received light level is high.
However, due to the different natures from one CD format to another, the frequency band in which the RF reproduction signal waveform is degraded also differs. The received light level of the reflected light, i.e., the reproduced RF signal, differs depending on the disc type. Nevertheless, it is necessary to ensure the compatibility of the disc playback apparatus with each format of disc.
In high-frequency compensation, the frequency and the amount of compensation may be set to one fixed value or may be set to a plurality of or continuously variable values. In either case, high-frequency compensation is mostly effective for press CDs or CD-R discs susceptible to level degradation of a high-frequency signal of, e.g., 3T to 4T.
On the other hand, high-frequency compensation may often be disadvantageous for CD-RW discs (of the recordable erasable type) susceptible to level degradation of a low-frequency signal of, e.g., 10T to 11T. It is therefore difficult to improve the quality of an RF reproduction signal of a CD-RW disc using only an RF equalizer of the related art designed only for high-frequency compensation.
Accordingly, a high-frequency compensation circuit of the related art may be effective for press CDs or CD-R discs, but may often be ineffective for recordable erasable CD-RW discs.